(1.) Field of the Invention
This invention relates to an improved batch-type fermentation process.
(2.) Description of the Prior Art
Commercial fermentations are commonly carried out by batch fermentation. To start a main batch fermentation, a seed culture is prepared in 2 to 3 stages of volume scale-up. To repeat a batch fermentation, a part of the fermented culture, if rich in viable cells, may be used as an inoculum for the next fermentation though it will reduce product yield. However, this sequence is not always preferred because the fermented culture often contains substances which inhibit the growth of microorganisms. Although harvested cells can be used instead, harvest and readdition of cells add to the cost of fermentation and these steps are less amenable to automation.
For example, in the brewing of beer, bittered wort is pitched with yeast of various moisture contents and is transferred to a vessel for batch fermentation. For true ales and stouts, Saccharomyces cerevisiae is used to ferment the wort for four to six days at about 15.degree.-20.degree. C., the yeast rising to the surface and being skimmed off later. The yeast may be reused as described above. For lager, Saccharomyces carlsbergensis is used to ferment the wort for from about ten to about twelve days at about 6.degree.-8.degree. C., the yeast and the lager being run-off into conditioning vessels.
As another example, in the production of penicillin, a large volume of concentrated, actively growing fungal suspension of Penicillium chrysogenum mold is required for the main fermenting tanks, to keep the fermentation time to a minimum. This is obtained in three stages. First, the selected culture is transferred from cold storage to a culture medium to produce an initial inoculum. This inoculum is then cultured in shake flasks to give a suspension. Finally, the suspension is grown in seed tanks in the plant for about 24-28 hours to the desired volume and concentration before transfer to the main fermenters. Fermentation is continued for three to five days, during which the vessel is cooled to keep the temperature between about 23.degree.-27.degree. C. and stirred and aerated with sterilized air. The introduction of large volumes of air causes frothing, which is controlled by the addition of antifoams. When fermentation is complete, the mycelium is removed on a rotary filter and the penicillin extracted into an organic solvent (such as butyl acetate or methyl isobutyl ketone), after acidification.
Fermentation of ethanol from carbohydrates is also now receiving attention as a future fuel and chemical feedstock. The use of ethanol as a gasoline additive is also increasing. A blend of gasoline with about 10% ethanol has been shown to increase the octane rating and to reduce emissions of nitrogen oxides and carbon monoxide. In North America, most industrial ethanol is currently made from ethylene derived from petroleum sources. However, as the price of ethylene is sharply increasing, production of fermentation ethanol will continue to rise. More efficient and economic technology of ethanol fermentation is thus desired. The industrial production of ethanol by fermentation thus demands efficiency of fermentation and low cost of substrates. Immobilization of ethanol-producing microorganisms would not only permit reuse of cells but would also accelerate ethanol production. Cells entrapped in polysaccharide gels have been most extensively studied.
Attempts have been made to avoid the problems and delays of start-up in batch type fermentations by the use of continuous fermentation. For example, some beer is brewed continuously in New Zealand. Also, some patents describe the use of continuous fermentations. For example, Amberg et al, U.S. Pat. No. 3,402,103 patented Sept. 17, 1968 taught a process in which spaced packing surfaces were disposed vertically in a tower, and fermenting organisms were grown on the packing surfaces. Thin films of carbohydrate-containing liquor were flowed substantially vertically downwardly without substantial change of direction over the organism surface so that the residence time of the feed liquor in the tower was less than about 30 minutes. Substantial conversion of carbohydrate to fermentation product, e.g. of the order of about 90%, was said to be achieved in one pass without recycling the liquor.
Griffith et al, in U.S. Pat. No. 4,127,447 patented Nov. 28, 1978 taught a continuous, biologically-catalyzed reaction with anaerobic microorganisms attached to a support in an upflow packed bed column. In the patented process, growth of the microorganisms was restricted to prevent the microorganisms from plugging the column by limiting the availability of an essential nutrient and/or by the presence of predatory protozoa which consume the anaerobic microorganisms. A membrane disruptive detergent was optionally provided in the column to lyse dead microorganisms to make them available as nutrients for live microorganisms.
Cheetham, in U.S. Pat. No. 4,393,136 patented Jul. 12, 1983 provided a continuous process for converting glucose, or other substrate to ethanol using immobilized bacterial cells under conditions which prevented growth of cells, e.g., by presenting the carbohydrate to the bacterial cells in a medium which is nutritionally inadequate for growth of such cells by lacking at least one factor required therefor.
Finally, Reed, in U.S. Pat. No. 4,506,012 patented Mar. 19, 1985 provided an improved process for preparing organic acids by a continuous homoacidogenic fermentation. This process provided increased volumetric productivity of the acid by employing a microorganism growing on the surface of a support material, e.g. activated carbon or corn cob granules.
However, continuous fermentations are not satisfactory solutions to avoid the problems of start-up of fermentations because continuous fermentations do not effeciently use the nutrients in the medium. Moreover, there is no effective simple way to reduce contamination in such continuous fermentation processes.
Previously, the present inventors have immobilized Saccharomyces cerevisiae cells onto epichlorohydrin triethanolamine (ECTEOLA)-cotton cloth to develop an ethanol-producing yeast film. In that method, a coiled yeast film placed in a cylindrical fermentor efficiently produced ethanol from sugars in the hydrolysate of Jerusalem antichoke tubers. Jerusalem artichoke represents a realistic source of ethanol in northern climates where it grows well and yields the highest amounts of carbohydrates (largely inulin) in its tubers. However, the preparation of ECTEOLA-cloth involves the use of unsafe chemicals and the cylindrical fermentor employed was not amenable to scale-up.